dc.description.abstract | The characterization of the relationship between a longitudinal response process
and a time-to-event has been a pressing challenge in biostatistical research. This
has emerged as an important issue especially in genetic studies when one
attempts to detect the common genes that govern both a longitudinal response
and a developmental event.
We considered the analysis of an ongoing study on Eucalyptus tree species by
Kenya Forestry Research Institute (KEFRI) grown in five different sites. These
species include four local landraces and six hybrid clones introduced from South
Africa. The study was designed to compare height and diameter at breast height
(DBH) measurements across the species and sites. The available data include
information on time to reaching 15 metres in height (50% to pole height) and the
repeated measurement of DBH fo the 10 species.
In this study we d~scribe a joint modelwhich accommodates incomplete repeated
measures and right-censored event times. Such an. approach allows us to account
for possible association between the time-to-event and the repeated
measurements, and permits the use of the repeated measures to increase the
efficiency of the estimate of the event-time process, We also compare the different
species and sites using multivariate analysis of variance (MANOVA) for repeated
measurements.
The species parameter estimates for the time-to-event components of the joint
model in general decreased in absolute order of magnitudes as compared with the
models assuming independence between the processes. Thus, the adjustment for
the evolution of DBH measurements resulted in less separation between the
clones and the local landraces.
Joint modeling also allows other characteristics of the repeated measurements
pattern, such as the rate of change (slope), to influence the risk of the event, the
hazard ratios approach one. This indicates that the distinction between the
species decreases if the random slope is added to the joint model. On the other
hand, adjustment for the event rates widened the differences between the DBH
measurements for GC642 and other species. | en |